The 3G Specification references given below are relevant to the teachings of this invention:
TS22.101 Chapter 4.3:
“3GPP specifications shall provide a mechanism which will enable a piece of user equipment (UE) to adapt to different radio interfaces as necessary and to determine the service capabilities available. The specifications shall also provide a mechanism which will enable a UE to select radio interfaces capable of providing appropriate service capabilities.”
TS22.228 (S1-010892)
7.2.1 Access Control
“The IM CN subsystem shall be able to verify at any time that the user is entitled to use the resources of the IM CN subsystem.
It must be possible to indicate to the terminal the reasons for denying access if this occurs, particularly when roaming. The following rejection scenarios must be supported
1. The serving network does not support IMS. The serving network shall indicate this fact to the terminal using an appropriate reject cause e.g, “No IMS available”.
2. The serving network supports IMS, but the terminal is not allowed IMS service in this network as there is no roaming agreement in place between operators The serving network shall indicate this fact to the terminal using an appropriate reject cause e.g, “IMS roaming not allowed”.
3. The serving network supports IMS, and roaming is allowed. In this case the terminal is allowed access to IMS, but it is still possible that service is rejected by IMS itself, e.g. due to failure of registration with the IMS.”
A 3GPP Release (Rel) 4/5 network is divided into a radio access network (RAN) and a core network (CN), which are connected via an open interface. The core network is, from a functional point of view, divided into a Packet Switched (PS) Domain, an IP Multi-Media (IM) Subsystem (IMS) and a Circuit Switched (CS) Domain.
It is possible to have network configurations in which the PS domain (through which IMS is available) exists without any IM subsystem (IMS), or for the user to not have access to the IMS for some reason (e.g., no IMS roaming agreement exists).
In a typical case the home network configuration is known by the mobile terminal. However, problems emerge in the roaming case, and when the mobile terminal is first switched on, when there are several operators or Public Land Mobile Networks (PLMNs) available.
For example, if the user desires to access and use IMS services, and before a Packet Data Protocol (PDP) Context Activation procedure has been executed in the selected PLMN, the mobile terminal does not know whether there is an IMS available for the user in the visited network configuration.
In summary, the existing problems include the following: (A) how does the mobile terminal determine that there is an IMS deployed in a PLMN; and (B) how does the mobile terminal know that the user has access to an IMS in a particular PLMN?
These problems can become especially troubling when the mobile terminal is roaming in a visited network area. Currently if there are, for example, three network operators, then in the worst case the mobile terminal or the user may select the “wrong” PLMN twice before selecting the PLMN that can provide the desired IMS services. This makes the selection procedure cumbersome for the mobile terminal/user, and further can require some undesirable amount of time to accomplish. This conventional procedure also needlessly consumes some amount of network bandwidth for making the unsuccessful IMS service inquiries.